Answer:
The option that combines the selected layer with the one immediately below it, without affecting other layers in the file, is "Merge Down".
Step-by-step explanation:
Flatten Layers combines all visible layers into a single layer, while Merge Visible combines all visible layers into a single layer, but keeps hidden layers separate. Merge Down, on the other hand, combines a selected layer with the one directly below it. So if you have Layer 1 and Layer 2 selected, Merge Down will combine Layer 2 with Layer 1. This is useful if you want to merge a specific layer with the one directly beneath it, while keeping other layers separate.